The invention relates to a device having at least one rotor which rotates or can rotate about a rotation axis, and having at least one magnetic bearing in which the rotor is borne or can be borne in a contactless manner.
U.S. Pat. No. 5,482,919 A discloses a device which has a rotor which can rotate about a rotation axis and has at least one superconducting winding (field coil) for an electric motor, and has a cryogenic cooler for cooling the superconducting winding. The superconducting winding may be formed from a known, metallic superconductor material (low-temperature superconductor) with a low critical temperature of Tc, below 35 K, such as a niobium-tin alloy or a ceramic metal-oxide superconductor material (high-temperature superconductor) with a high critical temperature Tc above 35 K, such as bismuth strontium calcium copper oxide, an yttrium barium copper oxide or a mercury or thallium compound. The cryogenic cooler makes use of rapid expansion of a working fluid (which is compressed by a compressor) such as helium, neon, nitrogen, hydrogen or oxygen for cooling in thermodynamic cycles (processes) such as a Gifford-McMahon cycle, a Stirling cycle or a pulse tube cycle. The superconducting winding is thermally conductively connected to a cold head, which rotates with the rotor, of the cryogenic cooler via two or more annular supporting elements composed of a material with a high thermal conductivity coefficient, and which are connected via heat pipes or thermally conductive rods. In this way, heat is dissipated from the superconducting winding by thermal conduction through a solid body to the cold head. There is no need for a liquid coolant such as liquid helium or liquid nitrogen in this known cooling system, so that there is also no influence on the rotation of the rotor from a cold liquid. The compressor of the cryogenic cooler can rotate with the rotor or may be in a fixed position with respect to the rotor, and may be connected to the cold head via a rotating coupling. U.S. Pat. No. 5,482,919 A states nothing more with regard to the bearing of the rotor.
Magnetic bearings are generally known for bearings for rotors, and allow the rotors to be borne in a contactless bearing, which is thus free of wear. Both active magnetic bearings with electromagnets and position control as well as passive magnetic bearings with automatic position stabilization are known.
DE 44 36 831 C2 discloses a passive magnetic bearing for bearing a rotor shaft with respect to a stator, which has a first bearing part which is connected to the rotor shaft, and a second bearing part which is arranged on the stator and surrounds the first bearing part. One of the two bearing parts has a high-temperature superconductor. The other bearing part has an arrangement of permanent-magnet elements arranged alongside one another and composed of a neodymium (Nd), Iron (Fe) boron (B) alloy or of a samarium (Sm) cobalt (Co) alloy. Adjacent permanent-magnet elements are magnetized with opposite polarity to one another. When a position change occurs, the permanent-magnet elements induce shielding currents in the superconductor, as a result of field changes. The resultant forces may be repulsive or attractive, but are always directed such that they counteract the deflection from the nominal position. In contrast to known active magnetic bearings, an inherently stable bearing can be achieved in this case, and there is no need for a complex control system that is subject to defects. The intermediate spaces between in each case two permanent-magnet elements are filled with ferromagnetic material in order to concentrate the magnetic flux, which emerges from the permanent-magnet elements, on the side facing the other bearing part. This results in a high level of bearing stiffness (stability, robustness). The permanent-magnet elements together with the ferromagnetic intermediate elements may be arranged axially with respect to the rotor shaft axis one behind the other in the form of thin rings, or else may be axially elongated and arranged one behind the other in the circumferential direction.
In a refinement of this known magnetic bearing, the permanent magnets are provided in a hollow-cylindrical arrangement on the inner bearing part, and the superconductor is arranged as a hollow-cylindrical structure on the inside of a hollow-cylindrical supporting body for the outer bearing part. Cooling channels are formed in the supporting body for passing liquid nitrogen through in order to cool the superconductor.
In another refinement according to DE 44 36 831 C2, the high-temperature superconductor on the inner bearing part is arranged on the rotor shaft, with a coolant channel being provided for the liquid nitrogen in the rotor shaft, in order to cool the high-temperature superconductor. This embodiment with a cold rotor body is proposed as part of a generator or of a motor with a cryogenic normally conductive or superconducting winding.
The document U.S. Pat. No. 5,214,981 A discloses a device for storing energy. This device has a rotating flywheel which has permanent magnets (which interact with stationary electromagnets) on its circumference for power transmission. The flywheel is borne in each case one magnetic bearing on opposite sides via two rotor shafts. In one embodiment (FIG. 1), one or more permanent magnets are provided in a cylindrical arrangement at each of the ends of the two rotor shafts. These ends project as first bearing parts into in each case one superconductor, in the form of a pot, as the second bearing part for the respective magnetic bearing. For cooling, the superconductors are each arranged in a cold bath of liquid nitrogen. In another embodiment (FIG. 3), each rotor shaft has a recess as the first magnetic bearing part on its end face facing away from the flywheel, with this recess being clad with a superconductor. The superconductor is cooled exclusively by the thermal radiation from the superconductor to the vacuum vessel, which is kept in a liquid bath filled with liquid nitrogen. Furthermore, the magnetic bearings have cylindrical second bearing parts, whose ends project into the recesses in the rotor shafts and have one or more permanent magnets in a cylindrical arrangement. The flywheel is enclosed together with the two magnetic bearings in a vacuum vessel which is evacuated to a pressure of less than 10xe2x88x924 Torr, in order to avoid friction of the rotating parts and the energy losses associated with such friction. The bearing gaps of the two magnetic bearings form continuous connections between the adjacent evacuated areas of the vacuum vessel.
JP 04370417 A and the associated abstract from Patent Abstracts of Japan disclose a further device for storing energy by a flywheel which is borne in two magnetic bearings and is arranged together with the magnetic bearings in a common evacuated vacuum chamber. Each magnetic bearing has a central permanent-magnet ring on the flywheel and two superconductor rings at an axial distance from it, which are arranged on stationary supporting disks, through which liquid coolant flows.
Finally, DE 197 10 501 A1 discloses an electrical machine having a stator with a polyphase winding for producing a rotating magnetic field, and with a rotor which rotates with the rotating field. The stator has a magnetic return path yoke, which forms a housing for the rotor. The rotor has a shaft which is passed through an opening, which is not sealed, in the housing and magnetic return path yoke. The rotor is composed entirely, or at least on its outside, of a high-temperature superconductor. Magnetic bearings for contactless bearing of the rotor are formed by the superconductor and by annular permanent magnets which are provided at two points. In order to cool the superconductor on the rotor, the entire machine is designed to have a small physical size and is operated completely in a cryogenic bath formed from liquid nitrogen.
Owing to the contactless bearing, the known magnetic bearings always have a continuous bearing gap, and gas and vapor can thus pass through them between the two sides which are connected by the bearing gap. Environmental air and moisture contained in it can thus enter the bearing gap, or can reach the rotor through the bearing gap. This results in the risk of the air humidity freezing on the cold components of the magnetic bearing or else of the rotor, if this is cooled, with such icing resulting in a restriction to operation, or even in damage to the magnetic bearing. Furthermore, the cooling processes with liquid coolant (cryogenic medium), in general liquid nitrogen, which are used exclusively for the superconductors of the described magnetic bearing according to the related art, are generally also subject to sealing problems in the region of the sensitive magnetic bearings, in addition to the problem of any moisture that has entered freezing on the cryogenic medium supply lines that are required, once again increasing the risk of icing or of other malfunctions of the magnetic bearing.
One aspect of the invention is based on the object of protecting the magnetic bearing or magnetic bearings for bearing of a rotor against such adverse effects on operation or damage.
The device accordingly has a rotor which rotates or can rotate about a rotation axis and at least one magnetic bearing, in which the rotor is borne or can be borne in a contactless manner (or without wear), and which has at least one superconductor (or: a superconducting structure).
The rotor is arranged together with the associated magnetic bearing or bearings in a common gas area (or gas-filled chamber), which is surrounded by a gastight wall. The rotor and magnetic bearings are thus, in other words, located in the same gas atmosphere, which is separated and shielded from the environmental air by the wall through which gas cannot pass. These measures result in the bearing gap of each magnetic bearing being filled with the gas with which the gas area is filled, and being protected by the gas area wall against the ingress of environmental moisture. Furthermore, pressure fluctuations, for example as a result of gas losses, can be tolerated within certain limits, since they affect all the components in the gas area in the same way.
As a further measure, a cooling device having at least one cold head, which is thermally coupled to the superconductor and dissipates heat from the superconductor mainly by thermal conduction as the heat transfer mechanism, is now provided for cooling the superconductor of the magnetic bearing or of each magnetic bearing. The use, as proposed, of a cold head (which in principle is known per se) for indirect cooling of the magnetic bearing is a considerably simpler solution in terms of design and handling than the direct cooling, as provided in the related art, via a liquid cooling medium. A cold head can easily be fitted to the magnetic bearing as a connecting piece for heat transmission. Furthermore, the use of one or more cold heads ensures deliberate cooling of the superconductor in the magnetic bearing and avoids the problems of the emergence (which can never entirely be avoided) of cryogenic liquid and the uncontrolled thermal conditions that result from this, with the risk of icing of the magnetic bearings as a result of freezing of residual moisture in the gas atmosphere or of moisture contained in the evaporated cryogenic medium.
The cooling device for cooling the cold head and hence for indirect cooling of the magnetic bearing or bearings preferably has a cryogenic cooler system which is operated in particular electrically and does not require the handling of cryogenic liquid gases in conjunction with the cold head. Different cold heads may in each case be connected to a dedicated cryogenic coolant, or else in any desired combinations to shared cryogenic coolers. Each cold head is preferably guided from the outside in a direction running essentially at right angles to the rotation axis to the superconducting structure of the magnetic bearing.
The magnetic bearing or bearings of the device generally has or have at least one inner bearing part and at least one outer bearing part, with the outer bearing part surrounding the inner bearing part and with a bearing gap, which runs around the rotation axis, being formed between the two bearing parts, and one of the two bearing parts is connected or can be connected to the rotor, in particular to its rotor shaft.
One of the two bearing parts of the magnetic bearing now preferably has at least one permanent magnet, while the other bearing part has the superconducting structure, which interact electromagnetically (by induction) with the permanent magnet or permanent magnets such that the bearing gap between the inner bearing part and the outer bearing part is formed or maintained. Where there are two or more permanent magnets, these are generally arranged alongside one another, in particular axially one behind the other with respect to the rotation axis and preferably in each case surround the rotation axis in a shape that is closed all round, in particular in the form of a ring, or else alongside one another in an arrangement which surrounds the rotation axis. The permanent magnet or magnets in one advantageous refinement surrounds or surround the rotation axis in a closed (all round) form, preferably in the form of a ring. The ring cross section may in this case in particular be circular, in the form of a disk or rectangular, corresponding to a hollow-cylindrical or toroidal ring shape. The ring longitudinal section at right angles to the rotation axis may thus, in particular, be in the form of a circular ring. Immediately adjacent permanent magnets are preferably magnetized with essentially opposite polarity to one another, at least on average, over domains which may be present.
One advantageous development of the magnetic bearing is characterized in that a flux concentrating element is in each case arranged between at least two of the permanent magnets, and/or a flux concentrating element is in each case arranged on the outside of the outer permanent magnets in the axial direction. Each flux concentrating element is used for conducting the magnetic flux of the permanent magnets and, in general, also for concentrating and amplifying it in the bearing gap and, for this purpose, is at least partially composed of a magnetically permeable material, in particular of a ferromagnetic material, for example iron (Fe).
The superconducting structure preferably surrounds the rotating axis in a closed form, in particular in the form of a ring, and/or essentially has a cylindrical shape, at least on the side facing the bearing gap. Furthermore, it is advantageous for the superconducting structure to be arranged on the side of the bearing part facing the bearing gap, in order to achieve good coupling efficiency.
In general, at least one cold head is in each case provided for each magnetic bearing, for cooling the superconducting structures and, expediently, the permanent magnets as well, in order to achieve a highercoercivity field strength.
The bearing gap of at least one of the contactless magnetic bearings is now preferably connected to the gas area and allows gas to be exchanged. In consequence, the bearing gap is located in the same gas atmosphere as that found in the gas area.
In general, the wall of the gas area is in a fixed position with respect to the rotor, that is to say its position relative to the rotation axis of the rotor remains unchanged during rotation of the rotor.
The rotor is preferably borne in the magnetic bearing via a rotor shaft which is connected or can be connected to the rotor. The rotor shaft is preferably passed to the outside through an opening, which is sealed by a rotation seal, in the wall of the gas area.
In one particularly advantageous embodiment of the device, the rotor is borne in at least two magnetic bearings, preferably via in each case one rotor shaft, which magnetic bearings are arranged on axially opposite sides of the rotor with respect to the rotation axis. The rotor is thus held in bearings on both sides and thus in a particularly robust manner.
One advantageous development of the rotor is characterized by at least one winding (coil) which generally runs around the rotation axis and is preferably formed by a superconductor.
Any low-temperature superconductors or high-temperature superconductors may be used as superconductors for the magnetic bearings and/or for the winding on the rotor. The superconductor may be a traditional low-temperature superconductor with a critical temperature up to 35 K, for example a metallic alloy such as a niobium tin alloy, or preferably a high-temperature superconductor with a critical temperature above 35 K, preferably above 77 K (i.e. the boiling point of nitrogen), preferably a metal-oxide or ceramic high-temperature superconductor such as bismuth strontium calcium copper oxide, yttrium barium copper oxide or a compound of mercury or thallium. The higher the critical temperature of the superconductor, the less energy is required for cooling.
Since high-temperature superconductors are, in particular, self-supporting only to a restricted extent, one advantageous development allows the superconducting structure of the magnetic bearing or the superconducting winding of the rotor to be arranged on or in a support or winding support. In order to cool the superconductor, the support or winding support preferably has a high thermal conductivity, for example being formed from metal.
In one particularly advantageous embodiment, the winding support of the rotor has a cavity (internal area) which extends axially with respect to the rotation axis. The winding can now advantageously be cooled in a space-saving manner via this cavity, in that a heat transmission unit in the cavity or on the cavity is thermally coupled to the winding support, preferably via a contact gas in the cavity.
The heat transmission unit is now preferably thermally coupled or can be thermally coupled to a cooling device for the rotor. This cooling device may be designed in a manner known per se, for example according to the initially cited U.S. Pat. No. 5,482,919 A, whose entire disclosure content is also included in the present application. The cooling device and/or heat transmission unit for the rotor may also operate with a liquid coolant such as liquid helium or liquid nitrogen, or else may have a cryogenic cooler system with at least one cold head, in the same way as the cooling system for the magnetic bearing.
According to one particular embodiment, the heat transmission unit has a preferably cylindrical heat transmission body which projects into the cavity in the winding support and between which and the winding support an intermediate gap is formed, which runs around the rotation axis and is filled with a contact gas. The heat transmission from the or cooling of the winding now takes place essentially by thermal conduction through the solid body and via the contact gas.
However, alternatively or additionally, cyclic vaporization and condensation of a heat transport gas, with appropriately chosen vaporization enthalpy, can also be used as the heat transport mechanism. The heat transmission unit may then, in particular, comprise a heat pipe.
In an embodiment which makes use of both the thermal transport mechanisms of thermal conduction and vaporization, the cavity in the winding support is at least partially filled with the heat transport gas, so that the heat transport gas is also used as the thermally conductive contact gas.
The cavity in the winding support and the intermediate gap between the heat transport body and the winding support can also be connected to the gas area in a manner which allows gas to be exchanged. This then results in a standard gas atmosphere inside and outside the rotor, and there is no longer any need to take any special measures in order to seal these gas areas.
The contactless arrangement of the heat transmission unit in the winding support is particularly advantageous when the two components are intended to be mechanically decoupled from one another, that is to say the heat transmission unit is intended to be fixed during rotation of the rotor. Such a fixed configuration of the heat transmission unit, which does not rotate with the rotor, and possibly of the connected cold head is expedient since there is no need to seal any rotating parts of the cooling system with respect to one another.
In one special physical development, at least one rotor shaft, which is borne in the associated magnetic bearing, is in the form of a hollow shaft. The hollow shaft can now at least partially accommodate the heat transmission unit and/or a connection between the gas area and an internal area of the rotor, in particular the cavity in the winding support
In order to protect the winding, it is preferably arranged in an internal area of a container in the rotor, which is preferably evacuated and is sealed from the rest of the gas area and from the cavity in the winding support.
The gas area of the device, in which the rotor and the magnetic bearing are arranged, is generally filled with a gas or a gas mixture which is used for thermal conduction for cooling of those components which need to be cooled and for this purpose makes contact with them and is therefore also referred to as the contact gas. This gas generally remains in the gas area throughout the operating life of the device. The contact gas is therefore in one advantageous embodiment an inert gas or a mixture of inert gases, with helium or neon being preferable, although nitrogen can be used for correspondingly high operating temperatures. Furthermore, in principle, hydrogen or oxygen are also suitable, although their handling is somewhat more problematic.
The gas with which the gas area is filled preferably contains virtually no water, or contains less than a critical amount of water, so that it is impossible for water to freeze on cold parts in the gas area. For this purpose, the gas is prepared with an appropriate purity, and/or is dried.
The gas pressure of the gas in the gas area in one advantageous embodiment is preferably at least as high as, and preferably higher than, the gas pressure in the outer area surrounding the wall of the gas area, in general atmospheric pressure. Even in the event of sealing problems or leakages in the area of the gas area wall, this reliably prevents the ingress of moist air and the possibility of ice being formed in consequence in the cold area.
The device is preferably used for electrical machines such as motors and generators.